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class MyLocker { |
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// Exclusive Write, which will wait for shared read to finish |
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// Shared read, which will wait for Exclusive Write to finish |
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public: |
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MyLocker() : mSharedUsage(0), mExclusiveWrite(false) {} |
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void ExclusiveWriteLock(); |
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void ExclusiveWriteUnLock(); |
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void SharedReadLock(); |
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void SharedReadUnLock(); |
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private: |
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mutex mMutex; |
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unsigned int mSharedUsage; ///< Each reader to increase it when reading so Writer will wait |
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bool mExclusiveWrite; ///< Each writer will wait for other writer to finish |
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condition_variable TellWriters; ///< Writers wait for other writers |
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condition_variable TellReaders; ///< Readers wait for Writers to finish, Writers wait for readers to finish |
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}; |
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void MyLocker::ExclusiveWriteLock() { |
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unique_lock<mutex> guard(mMutex); |
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if (mExclusiveWrite) { |
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// Wait for other writers to finish |
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TellWriters.wait(guard); |
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} |
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// I am the sole writer now |
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mExclusiveWrite = true; |
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if (mSharedUsage != 0) { |
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// wait for all readers to finish |
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TellReaders.wait(guard); |
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} |
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// Sole Writer, no readers, Full EXCLUSION |
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} |
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void MyLocker::ExclusiveWriteUnLock() { |
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{ |
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// reduced scope to destory the guard as soon as possible |
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// and before notifying all readers and writers |
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unique_lock<mutex> guard(mMutex); |
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mExclusiveWrite = false; |
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} |
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// tell all readers and writers to resume operation |
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TellWriters.notify_all(); |
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TellReaders.notify_all(); |
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} |
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void MyLocker::SharedReadLock() { |
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unique_lock<mutex> guard(mMutex); |
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if (mExclusiveWrite) { |
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// wait for writers to finish |
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TellWriters.wait(guard); |
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} |
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// I will now be writing |
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mSharedUsage++; |
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} |
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void MyLocker::SharedReadUnLock() { |
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{ |
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// reduced scope to destory the guard as soon as possible |
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// and before notifying all readers and writers |
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unique_lock<mutex> guard(mMutex); |
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SharedUsage--; |
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} |
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// done reading, tell all to resume |
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TellWriters.notify_all(); |
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TellReaders.notify_all(); |
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} |
